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Atomic Scale Mechanisms of the Reduction of Nickel–Magnesium Aluminate Spinels


  • Based in part on the thesis submitted by B. E. Hill for the Ph.D. degree in ceramic engineering Alfred University, Alfred, NY, 2012.


The dynamics of the reduction reaction of NixMg1−xAl2O4 to form nickel metal and a remnant oxide was quantified to understand spinel behavior in catalysis applications. X-ray diffraction, thermogravimetry, and pycnometry were employed to track the evolution of high-Ni spinels to metastable nonstiochiometric spinels during reduction, but before the phase transformation to theta alumina. Rietveld refinements of X-ray diffraction data were used to quantify structural changes in the spinel and the phase fraction, crystallite size, and microstrain of all phases during H2 reduction. During reduction, one O2− is lost for each Ni2+ reduced to Ni metal. Ni0.25Mg0.75Al2O4 and Ni0.5Mg0.5Al2O4 were shown to form Ni metal and a non-stoichiometric spinel of the same Mg-Al ratio as the starting composition. NiAl2O4 and Ni0.75Mg0.25Al2O4 were found to become unstable as full reduction was approached, and metastable spinel, Θ-Al2O3, and α-Al2O3 formed sequentially given sufficient time at temperature.

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